In the case of devices in which arcing is generated between electrodes in the device, an insulator is disposed in the vicinity of the arcing in order to protect the other parts of the device from the heat and ultraviolet-rich light from the arcs. These devices can be exemplified by circuit breakers in which arcing is extinguished by spraying sulfur hexafluoride gas (SF6 gas) between the electrodes and arc chute breakers in which a magnetic field is applied by coils disposed on both sides of the electrodes and arcing is extinguished by blowing sulfur hexafluoride gas into a region known as the arc chute.
Fluororesin not filled with another substance was used for this protective insulation in the past. However, when an unfilled fluororesin was used, the heat and ultraviolet-rich light from the arcing generated during circuit interruption produced an electrically conductive carbonized material not just on the surface of the resin, but also in the interior of the resin, causing a substantial reduction in the insulating performance.
In addition, gas generated by the resin's interior carbonization was discharged from the interior, causing ejection of the fluororesin. This resulted in the formation of substantial unevenness in the surface of the resin and a substantial decline in the mechanical strength of the insulator disposed for purposes of protection. Another problem was that the unevenness formed in the resin's surface resulted in a poor gas flow when the arc-extinguishing gas was blown in and prevented an adequate cooling effect from being obtained.
In response to these problems, Patent Document 1 proposes that the fluororesin be filled with boron nitride; this reflects the light from the arc and prevents its penetration into the interior of the resin and thereby prevents interior deterioration. Due to its filling with boron nitride, this insulator is whiter than the unfilled insulator and also has a higher light reflectance. However, a punctiform discoloration and color heterogeneity not seen for the boron nitride-free fluororesin by itself have been produced in some instances due, inter alia, to filling with a mixture of boron nitrides from different production lots and the residence in the resin of gas produced from the boron nitride during resin baking.
These discolored regions absorb heat and ultraviolet radiation from the arcing more readily than other regions, which, being white, have a higher reflectance, and this can cause local deterioration. Since as a consequence it must basically be ensured that an insulator that exhibits substantial color heterogeneity and/or regions of discoloration is not used, quality control becomes more problematic than for an insulator not filled with boron nitride, which drives up the cost of quality control and makes it difficult to improve production efficiency.
In addition, while materials obtained by loading a fluororesin with 10 to 20 weight % of boron nitride have entered into widespread use at the present time as insulators for circuit breaking applications, since boron nitride has a high thermal conductivity fluororesin loaded with 10 to 20% boron nitride ends up having a high thermal conductivity, which has led to the additional problem of an elevated heat-induced wear of the fluororesin.
The addition of molybdenum disulfide MoS2 is proposed in Patent Document 2 in order to absorb the heat and ultraviolet-rich light from the arcing in the surface layer and thereby prevent interior deterioration. This method can almost entirely eliminate color heterogeneity and discoloration due to its blackening effect. However, both MoS2 and carbon, which is widely used as a black pigment, are electroconductive materials, and as a consequence the insulating performance of the insulator and its arc resistance are reduced even when the filling rate is controlled.
The addition of 0.2 to 5 weight % of an inorganic pigment having a particle diameter no greater than 1 μm and/or organic pigment having a particle diameter of 0.5 μm is proposed in Patent Document 3. However, there is no stipulation of the resin/pigment combination, and the technology shown in Patent Document 3 (for example, filling a polytetrafluoroethylene resin, which requires baking at around 300° C., with an organic pigment having a heat resistance of only hundred and several tens degrees centigrade and/or with ultramarine blue pigment, which gradually discolors at temperatures of 300° C. and above, is taught therein) cannot solve the problem of color heterogeneity and discoloration, which is one of the problems that the present invention is directed to solving.    Patent Document 1: Japanese Patent Publication No. Hi-37822    Patent Document 2: Japanese Patent Application Laid-open No. H10-172400    Patent Document 3: Japanese Patent Publication No. S62-60783    Non-Patent Document 1: “Colorant Engineering Handbook”, Japan Society of Color Material, Asakura Publishing Co., Ltd., 1989